Momentum - which was born out of Mr Corbyn's first Labour leadership campaign - is a vast grassroots organisation that campaigned for Labour across the country.

It used an app to direct activists to campaign in marginal seats and deployed social media campaigns to win support - but declared spending of just £38,742 ahead of June's general election.

The Electoral Commission said its probe would look into whether Momentum's spending broke legal limits for non-party campaigners in national elections.

Momentum - which was born out of Jeremy Corbyn's first Labour leadership campaign - is a vast grassroots organisation that campaigned for Labour across the country

Momentum, lef by Jon Lansman (pictured) used an app to direct activists to campaign in marginal seats and deployed social media campaigns to win support - but declared spending of just £38,742 ahead of June's general election

And it will also consider whether returns submitted by the group, founded in 2015 as a grassroots movement to support Mr Corbyn's leadership of the Labour Party, accurately recorded donations and payments relating to the 2017 campaign.

The Commission's director of political finance and regulation and legal counsel, Bob Posner, said: 'Momentum are a high-profile active campaigning body.

'Questions over their compliance with the campaign finance rules at June's general election risks causing harm to voters' confidence in elections.

'There is significant public interest in us investigating Momentum to establish the facts in this matter and whether there have been any offences.

'Once complete, the Commission will decide whether any breaches have occurred and, if so, what further action may be appropriate, in line with its enforcement policy.'

ELECTION WATCHDOG HAS FOUR QUESTIONS FOR MOMENTUM

The Commission stated that its investigation will look at whether or not Momentum:

Spent in excess of the spending limits;

Submitted a return that did not include accurate donation information;

Submitted a return that was not a complete statement of payments made in respect of controlled expenditure; or

Submitted a return that did not include all invoices for payments of more than £200.

Electoral law imposes strict spending limits on non-party campaigners seeking to influence people to vote for one particular political party or any of its candidates.

Spending is limited to £31,980 in England, £3,540 in Scotland, £2,400 in Wales and £1,080 in Northern Ireland for the regulated period, which in this year's case stretched for 12 months before the June 8 ballot.

Founded by Jon Lansman as a means of bolstering Mr Corbyn's leadership at a time when he was seen as being under threat from centrist MPs and activists, Momentum has become increasingly influential within the Labour Party.

It boasts a network of more than 23,000 members, 150 local groups and 200,000 supporters, many of whom took part in campaigning around the 2017 election.

Electoral Commission records show that Momentum reported total spending of £38,742.54 across all four parts of the United Kingdom during the general election campaign, £257.46 below the £39,000 limit.

The movement is a standalone organisation but is influential inside Labour, endorsing candidates and running a parallel conference (pictured is a pro-Corbyn protest in Westminster)

The watchdog added: 'It is possible that during the course of the investigation, the Commission will identify potential contraventions and/or offences under PPERA (Political Parties, Elections and Referendums Act 2000) other than those set out above.'

A Momentum spokesman said: 'Much of the Electoral Commission investigation refers to a series of administrative errors that can be easily rectified.

'Momentum put a lot of effort and resources into detailed budgeting and financial procedures during the election to ensure full compliance.

'Our election campaign was delivered on a low budget because it tapped into the energy and enthusiasm of tens of thousands of volunteers across the country.

'We have a good working relationship with the Electoral Commission, and will fully comply with the investigation going forward.'

How a former public schoolboy is hijacking Labour: Jon Lansman founded Corbyn's Momentum which rails against the rich... yet lives in a luxury building and his family run a property firm with links to a tax haven

By Guy Adams for the Daily Mail

Sited in a prime spot on London’s South Bank, with sweeping views of Tower Bridge, the famous Le Pont de la Tour restaurant downstairs, and a 24-hour concierge to service high-powered residents, Butler’s Wharf is one of Britain’s most exclusive apartment blocks.

The Thames-side landmark, where a four-bed penthouse is on sale for £6.25 million, was converted from a warehouse into high-end flats in the Eighties, and has provided a pied-a-terre to assorted City bankers, top lawyers and plutocrats ever since.

Strangely, this symbol of unfettered turbo capitalism, where homeowners pay around £4,000 a year in service charges, has for the past two years been the official home of a far-Left organisation dedicated to the destruction of such conspicuous wealth.

The privately-wealthy 60-year-old filed paperwork setting up the group where he lived at Butler's Wharf

For according to Companies House, a spectacular first-floor flat at Butler’s Wharf is headquarters to the notorious pro-Corbyn pressure group, Momentum.

In one of those bizarre ironies you probably couldn’t make up, the organisation, whose 30,000 members style themselves as a sort of neo-Marxist ‘praetorian guard’ of Labour activists, has the exclusive yuppie property as its registered address.

The reason for this awkward fact is one Jon Lansman, a veteran political organiser and a fixture on the hard Left of the party for four decades, and who founded Momentum to support his chum Jeremy Corbyn’s bid for the leadership in the summer of 2015.

Like champagne socialists of the old school, the privately-wealthy 60-year-old decided to file paperwork setting up the group from where he lived: a state-of-the-art bolthole at Butler’s Wharf.

So it goes that Momentum (whose offices are a mile away in a grotty bit of Aldgate) remains formally registered to an extraordinarily opulent property which, according to estate agent’s particulars, boasts, among other things, ‘extra-wide oak floorboards’, ‘remote control dimmable lighting’, exposed beams and brickwork, a ‘walk-in style walnut wardrobe’ and a ‘large made-to-measure Italian couch’.

Quite how the group, whose army of sometimes very abusive activists have played a crucial role in Corbyn’s electoral success, squares this fact with its endless tub-thumping on such topics as wealth inequality and the housing crisis is anyone’s guess.

But it doesn’t seem to have done Lansman, who is Momentum Chairman, much harm.

For this veteran fixer, little known outside the Westminster bubble, is about to become one of the most powerful figures in British politics. Last week, the portly, bearded grandfather announced he’s to stand for Labour’s ruling National Executive Committee (NEC).

Jon Lansman pictured with Tony Benn, a darling of the left from a privileged background who supported Fidel Castro

Thanks to a recent rule change — which he himself successfully lobbied for! — an extra three members of the powerful committee (which also has representatives from Parliament and Trade Unions) are about to be selected via a ballot of Labour members and local branches.

Lansman is one of four candidates that Momentum hopes to insert in these positions. And such is the campaigning muscle that both he and his organisation holds among the party’s grassroots that he is almost guaranteed to win.

At this stage, readers may be thinking ‘so what?’

But while internal Labour Party politics can often seem rather arcane, this particular development will have ramifications far beyond Westminster.

The reason is this: since 2015, the NEC, which runs the party machine and oversees all policy-making, has been delicately balanced between moderates and Left-wingers loyal to Momentum, allowing it to occasionally check the excesses of Corbyn.

But that is set to change. For the expected election of Lansman and two allies will give the hard-Left complete control, thereby completing the Corbyn-isation of the official Labour machine.

To political analysts, this represents a classic example of ‘entryism’, a technique by which a well-organised lobby group — usually with extreme views — plots to get its members to join a mainstream political organisation en masse to subvert its policies and expand their influence.

Militant Tendency’s takeover of the Liverpool Labour Party in the Eighties was classic entryism. More recently, the Unite trade union flooded a local Labour Party in Scotland with supporters to try to get its hard-Left candidate to be selected to fight to become an MP for the constituency.

Lansman is an expert in ‘entryism’, having played a prominent role in unsuccessful efforts by militant socialist organisations to infiltrate Labour in the Eighties.

Back then, he was heavily involved in the Campaign for Labour Party Democracy, which lobbied (sometimes rather viciously) for members to be able to de-select sitting MPs who they deemed insufficiently Left-wing.

Today, Corbynistas are up to similar tricks, for example orchestrating malevolent campaigns against moderate Labour MPs such as Angela Eagle, whose constituency office windows were smashed.

But this time, there’s a big difference: unlike in the Eighties when their campaign to radicalise Labour failed, they are now on the brink of victory.

‘After decades in the wilderness, the promised land is not far off,’ is how the Momentum founder explains his hopes on his Twitter page.

If elected to the NEC and able to wield huge influence, Lansman would become Labour’s puppet-master-in-chief: a man able to manipulate almost every aspect of its affairs — from its election manifesto to who can stand as MPs.

All of which begs two questions: who exactly is Jon Lansman? And how, in two years, has the Left-wing activist from a privileged background risen from nowhere to represent the power behind Corbyn’s throne?

Let’s start by taking a peek through his gilded keyhole.

In April 2015, his rambling house in London’s fashionable Shoreditch (from where he moved to Butler’s Wharf) was placed on the market.

Priced at just over £1.5 million, pictures on the estate agent’s website show the three-bedroom pad, which was decorated with modern art, had a glass staircase, Rayburn cooker range, brushed steel kitchen, roof terrace, ‘insulated pantry,’ basement art studio, and ‘a spectacular wine cellar with a 2,200 bottle capacity’.

Yet look closely, and clues to Lansman’s political allegiances were also visible. In the living room, underneath a flat-screen TV, was a large bust of Karl Marx.

Bookshelves were stacked with texts about Lenin, Trotsky and Stalin, along with books by a political mentor, Tony Benn. In the magazine rack next to the reclaimed wood kitchen table were copies of a Left-wing magazine.

To old acquaintances, the home (which, incidentally, was bought by the pop star Emeli Sande) summed up its owner, a naturally gregarious character with an appalling dress sense (he occasionally steps out in dungarees) whose views have barely shifted since he first joined Labour in the mid-Seventies.

In Lansman's living room underneath a flat screen TV were books about Leon Trotsky and other left-wing thinkers

‘Jon calls himself a Bennite, and, like Tony, sees nothing wrong with being very Left-wing at the same time as being filthy rich,’ says one.

‘If you wanted to tease him, you could call him a “gentleman communist”. He’s a man of contradictions, which is a nice way of saying he’s a bit of a hypocrite. I mean, it’s very easy to bang the drum for massive income tax hikes when you have enough squirrelled away to be comfortably off, regardless.’

Like many of Corbyn’s inner circle, Lansman was educated at a private school (Highgate, where fees are currently £20,370 a year). In addition to Momentum, he runs a blog called Left Futures, which often rails against ‘crooked capitalism’ and has carried articles about the ‘housing crisis’, including a recent tirade by Labour’s Diane Abbott about the ‘insecurity so many face in Tory Britain’.

Intriguingly, however, Left Futures is owned by a company called Ortonovo Holdings, of which Lansman and his 26-year-old son, Ben, are the sole directors. Its registered office, in Soho, is an address also used by a Lansman family business, called Foundation Property & Capital (FP&C). This real estate firm was set up in the early Nineties with Jon’s father Bernard as a director. Today, it’s run and owned by Jon’s brother Stephen.

Describing itself as ‘unashamedly opportunistic,’ the property firm controls a complex web of 32 companies, all of which have Jon’s son Ben and brother Stephen (who has clocked up 54 company appointments) as directors.

Clues to Lansman’s political allegiances were also visible in the living room, where bookshelves were stacked with texts about Lenin

According to the FP&C website, its interests include a ‘nationwide portfolio’ of former Little Chef restaurants which are now ‘asset-managed and leased to McDonald’s’ along with notorious tax-avoiding firm Starbucks.

All of which seems a little rum, given that Momentum campaigns against the ‘appalling working conditions’ at the fast food giant McDonald’s and has called for a ‘McStrike’ of workers.

Other ‘opportunistic’-sounding recent FP&C ventures include advising on the sale of a homeless hostel in Central London for £22.5 million, with the loss of 170 beds. In 2015, it was reported to be advertising ‘tax efficiency’ opportunities for the rich, including the ability to ‘shelter income tax for the first two years of investment,’ and was shown to have once struck a deal that involved using a ‘special purpose lending vehicle’ — effectively a subsidiary company — set up in the tax haven of Luxembourg.

None of this is illegal, of course, but it sits unhappily with Momentum’s long-standing campaigns for ‘tax justice’, not to mention the ‘fat-cat tax dodgers’ condemned by Lansman’s blog Left Futures, or Corbyn’s commitment to crack down on tax-dodging members of the ‘one per cent’ who seek to shelter cash offshore.

Asked about this contradiction, a friend of Lansman states: ‘The company isn’t owned by Jon, but by his son and brother. Jon doesn’t have any part in the day-to-day running of the company.’

Meanwhile, another ally vigorously denies that Lansman and his family’s wealth is anything to be ashamed of.

‘People might call him a hypocrite, but it’s actually the other way round,’ he argues. ‘Under a Corbyn government, people like Jon will almost certainly pay far more tax than they do now, so as a privileged person pursuing Left- wing politics, one can argue that he’s doing something very principled.’

Be that as it may, Lansman’s wealth and background is the subject of carping on the Leftist extremes of the Corbyn movement, where (as a Jewish atheist) he has also managed to make enemies by speaking out against anti-Semitism in the Labour Party.

Lansman has also (again to his credit but to the dismay of many acolytes) called for Momentum supporters to cease ‘personal attacks and harassment’ which have led some centrist Labour MPs to fit panic alarms in their offices and homes, and prompted the BBC to hire a security guard to protect its political editor Laura Kuenssberg at the Labour conference in Brighton.

Elsewhere, far-Left critics complain that while Momentum is supposed to harness the power of grassroots Corbynistas, its parent company Momentum Campaign (Services) Ltd — and therefore all of its valuable data — is entirely owned by Lansman and an associate called Simon Fletcher.

This is seen as ironic by those who have followed his career since the early Eighties, when he spent years in the Campaign for Labour Party Democracy, arguing that members should control all key assets and decisions.

The son of a successful businessman who ran a string of clothing shops, Jon Lansman was raised in leafy Cockfosters, North London, where homes now cost upwards of £1.2m, and converted to socialism after spending time on a kibbutz in Israel, aged 16.

At Cambridge University, he devoted much time to student politics, standing unsuccessfully in elections. Then shortly after graduating with a lower second-class degree in Maths and Economics, he took an unpaid job running Tony Benn’s 1981 bid to replace Denis Healey as Labour’s deputy leader.

A darling of the Left, Benn (from an aristocratic background and indeed the erstwhile Viscount Stansgate) promoted extra-parliamentary direct action and civil disobedience, backed unilateral nuclear disarmament and nationalisation, supported the miners’ strike and Castro’s Cuba and wanted to take the Queen’s head off postage stamps.

The bitter campaign saw Lansman in the news when Healey wrongly claimed that the Bennite had orchestrated far-Left protesters to disrupt a speech he had given in Birmingham.

In fact, Lansman had been of miles away in Wales, visiting the family home of his lawyer girlfriend. Healey was forced to apologise.

Shortly afterwards, the News of the World newspaper published a colourful article about this ‘rising red star’ — revealing his ‘silver spoon life’.

It quoted his father Bernard who was chairman of the Hackney Conservative Association (and had stood for election to the Greater London Council) commenting that his son frequently moaned about having been sent to a leading public school.

‘My son often laments that he missed the benefits of a secondary education, but I doubt he’d have ended up at Cambridge unless he’d gone to Highgate,’ he observed.

After Benn narrowly lost out to Denis Healey, pictured, Lansman disappeared from public view and later set up Momentum

Lansman senior also said he was mystified how his class warrior son could possibly advise Tony Benn on industrial policy. ‘I can’t understand how Jon can talk about the workers and unemployment,’ he said. ‘The only time he’s ever worked was a part-time job to do with driving.’

In the event, Benn narrowly lost out to Denis Healey, and Lansman disappeared from the public eye.

After taking a job as business manager for Tribune, the Labour newspaper (only for moderate trade unions to withdraw advertising on grounds that it had been taken over by ‘a bunch of Left-wing lunatics’), he moved into management consultancy. Then, after marrying a council worker called Beth Wagstaff, the couple moved to Hertford.

As well as Ben, the couple had another son, Max, now a lawyer, and a daughter, Molly. But in 1994, tragedy struck: Beth was diagnosed with breast cancer. In 1999, she died. Devastated, Lansman later received a six-figure compensation payment from the NHS, which due to a hospital blunder had failed to correctly diagnose the disease. He quit work and dedicated himself to raising the children.

‘That experience really made me lose my interest in the world of work,’ he later recalled. ‘It destroyed any ambition I had.’

It wasn’t until 2010, when his daughter had left school, that he felt ready to return to politics, selling the family home, moving to London and becoming an aide to Labour MP Michael Meacher.

Another Bennite, the MP campaigned for the renationalisation of industry, an end to private education and for workers’ co-operatives to control much of Britain’s infrastructure. In private, Meacher owned eight homes and once unsuccessfully tried to sue a journalist who mocked him for being middle class.

Meacher’s death in 2015 left Lansman again pondering mortality (that summer, in an act of extreme generosity, he donated a kidney to a complete stranger). But then Jeremy Corbyn, an old chum, launched a bid for the Labour leadership.

The rest is history: Lansman set up Momentum — using those classic entryist techniques that could have come out of a Trotskyist textbook — and Corbyn, the most Left-wing leader in Labour’s history, won a surprise victory.

For his part, Lansman has inveigled his way into a position of huge power, controlling an army of idealistic class warriors . . . from one of London’s smartest addresses.